The present invention relates to a compact zoom lens suited for video cameras, digital still cameras equipped with a solid-state imaging element such as a CCD. More specifically, the present invention relates to a compact zoom lens suited for video cameras and digital still cameras equipped with a solid-state imaging element with a high pixel count.
In recent years, there have been significant technical advances in solid-state imaging elements such as CCDs used in digital still cameras and video cameras, resulting in higher densities and increased pixel counts. Accompanying this, there is a demand for lenses with superior optical properties. Also, with the increased compactness of digital still cameras and video cameras, there is a strong demand for zoom lenses mounted on these devices for imaging to be compact and light-weight.
In order to provide good portability, the need for compactness especially involves shortening of the lens when the device is being carried, i.e., the lens can be thinner when stored.
In order to make efficient use of light, microlenses are formed on the surface of the solid-state imaging element such as the CCD. As a result, if the angle of incidence of light to the solid-state imaging element is too high, vignetting (so-called “shading”) can take place, resulting in light not entering into the solid-state imaging element. As a result, the lenses used with the solid-state imaging element is desired to be positioned so that the exit pupil can keep an adequate distance from the image plane and a telecentric optical system, in which the incident angle to the solid-state imaging element, i.e., the exit angle, can be kept small. With improvements in microlenses that have accompanied with the technical advances of recent years, incident angles of up to around 15 degrees can be used with no problems.
For conventional zoom lenses with zoom factors of about 2 or 3, many examples of zoom lenses, such as those mounted on a compact camera, have been disclosed where two lens groups are used (e.g., see Japanese Laid-Open Patent Document Number 2003-075721 and Japanese Laid-Open Patent Document Number 2003-307676). These zoom lenses are so-called telephoto-type two-group zoom lenses formed from a first lens group having a positive refractive power and a second lens group having a negative refractive power. This design is advantageous in a point that it allows the lens to be thinner when stored.
However, in this type of arrangement with a first lens group having a positive refractive power and a second lens group having a negative refractive power, the exit angle of the outermost light rays becomes too large especially at the wide-angle end. As a result, using this design for solid-state imaging elements such as CCDs is extremely difficult.
In another known arrangement, a zoom lens with a two-group structure includes, going from the object side to the image plane side, a first lens group having an overall negative refractive power, and a second lens group having an overall positive refractive power (e.g., see Japanese Laid-Open Patent Document Number 3000-035537).
However, since the first lens group is formed from four lenses having a positive refractive power, a negative refractive power, a negative refractive power, and a positive refractive power, the total length of the camera is long when it is stored (retracted). Also, since the lens furthest to the object side has a positive refractive power, even it is effective for distortion correction, this results in providing a large outer diameter for the lens furthest to the object side, making a thinner, more compact design difficult.
With two-group zoom lenses such as those described above, the exit angle of the outermost rays, especially at the wide-angle end, becomes large, which prevents telecentricity and these structures from being used in recent solid-state imaging elements with high pixel counts. Also, with conventional two-group zoom lenses, the first lens group is formed from a large number of lenses which increases the number of parts. This makes it difficult to achieve a compact, light-weight design. In particular, a thin design cannot be achieved when the camera is stored.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.